Containers used to hold food, beverages, pharmaceuticals, nutraceuticals, etc. may undergo a retort sterilization process. In such a process, a container is filled with product (e.g., food) at an elevated temperature and then sealed. After sealing, the container is further heated to sterilize the contents of the container during the retort process. Changes in temperatures and pressure during the retort process cause the container to experience a variety of positive and negative differential pressures during the process. For example, heating of the sealed container increases the internal pressure of the container. During the retort process, the pressure within the container can reach levels in excess of 30 psig. Following sterilization the container cools and the internal pressure of the container decreases and may dip to below an ambient pressure external to the container. Containers often subjected to a retort process include, for example, metal containers, such as double seamed metal cans. Metal cans are designed to withstand the significant positive and negative gradient pressures that may be experienced during a retort process. For example, side walls of metal cans may include ridges encircling the can, known as beads, which aid in the prevention of deformation or paneling when the can experiences a negative differential pressure. Also, the metal cans are designed to withstand high pressures without bulging. The mechanical properties of the metal used for a metal double seamed can are not significantly affected by the temperature increase employed during a retort process.
In comparison to the material of a metal double seam can, polymers used to make polymer containers are much less strong than materials used to make a metal double seam can (e.g., stainless steel), specifically, they have lower elastic modulus and are softer. Also, the mechanical properties of polymers used to make a polymer container are greatly affected by the temperature increase that would be employed during a retort process. For example, the tensile modulus of polypropylene (PP) changes by a factor of about ten between 20° C. and 130° C.
For polymer containers, the high temperature of the retort process and accompanying pressures may cause undesirable creep deformation of the container. Further, the negative differential pressure during various portions of the retort process may also cause geometrical deformation of the container shape. Such deformations may be described as bulging of the container or paneling of the container.